The present nonprovisional application claims priority under 35 USC 119 to Japanese Patent Application No. 2002-003616 filed on Jan. 10, 2002; Japanese Patent Application No. 2002-008962 filed on Jan. 17, 2002 and Japanese Patent Application No. 2002-008964 filed on Jan. 17, 2002 the entire contents thereof are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a method of and a system for controlling the charging of a secondary battery, and particularly to a method of and a system for controlling the charging of a secondary battery, which are suitable for enhancing the durability of the secondary battery, that is, keeping the initial charging/discharging performance of the secondary battery for a long-period of time.
2. Description of Background Art
In the case of charging a secondary battery using nickel oxide as the material of a cathode (positive electrode), for example, a nickel-cadmium battery or a nickel-hydrogen battery, as the battery is charged to a full-charge level, oxygen gas is generated at the positive electrode. If the generation of the oxygen gas is continued, the inner pressure of the secondary battery is raised. Accordingly, the oxygen gas thus generated is required to be consumed by local cell reaction on the surface of an anode (negative electrode). In general, taking into account the gradual reduction in capacity of the negative electrode by the local cell reaction, the capacity of the negative electrode is set to be larger than that of the positive electrode. For example, a ratio (NP ratio) of the capacity of the negative electrode to that of the positive electrode is set to a value in a range of about 1.65 to 2.0. However, since the charging capacity of the battery is increased with the positive electrode taken as a rate-limiting factor, the increased capacity of the negative electrode brings enlargement of the secondary battery but does not contribute to an increase in charging capacity of the battery.
A charging method capable of suppressing the generation of oxygen gas by stopping the charging at a charge level less than a full-charge level, for example, 97% of the full-charge level has been proposed in Japanese Patent Laid-open No. Hei 5-111175. This charging method is advantageous in that since the generation of oxygen gas is suppressed, the reduction in capacity of a negative electrode by local cell reaction can be prevented, with a result that it is possible to reduce the capacity of the negative electrode, and hence to miniaturize the secondary battery, that is, increase the charging capacity.
The charging control to stop the charging at a charge level less than a full-charge level as described above is preferable from the viewpoint of the durability of a secondary battery; however, if the charging is usually stopped at a charge level less than the full-charge level, a state wherein several percent of electrode plates are not used is repeated. As a result, the unused electrode plates are deteriorated, which do not contribute to the charge amount.
By setting a voltage reference value (cut-off voltage) representative of a reference charge amount at which the charging is to be stopped, at a level as close to a full-charge level as possible, it is possible to reduce the number of the electrode plates not contributing to the charge amount. However, since the voltage of a secondary battery is dependent on a battery temperature and an environmental temperature, it is not easy to accurately detect that the voltage of the secondary battery reaches the cut-off voltage, and therefore, it is not expected to accurately control the charging of the secondary battery.
In view of the foregoing, the present invention has been made, and an object of the present invention is to provide a method of and a system for controlling the charging of a secondary battery, which are capable of preventing a reduction in full-charge amount due to repetition of the changing at a charge level less than the full-charge level.
It may be considered to periodically charging the secondary battery at a charge level more than the full-charge level. However, if the charging of the battery to a charge level more than the full-charge level is simply performed as conventional, there may occur a difference between the charging of the battery to a charge level less than the full-charge level and the charging of the battery to a charge level more than the full-charge level. Such a difference in charging time therebetween gives an unfavorable feeling to a user.
In view of the foregoing, the present invention provides a method of and a system for controlling the charging of a secondary battery, which are capable of preventing a reduction in a full-charge amount due to repetition of the changing at a charge level less than the full-charge level, and performing a simple control by means of which the charging time is usually kept nearly constant.
To achieve the above object, the present invention provides a method of controlling the charging of a secondary battery, including the steps of: controlling the charging of the secondary battery by using both a first charging control specified to stop the charging at a charge level less than a full-charge level and a second charging control specified to stop the charging at a charge level equal to or more than the full-charge level; wherein after the charging by the first charging control is continuously repeated by a specific number of times, the next charging is performed by the second charging control and if the state of the secondary battery satisfies a specific refresh discharge reference in the second charging control, the secondary battery is discharged before the charging of the secondary battery.
According to a second feature of the present invention, there is provided a system for controlling the charging of a secondary battery, including: first charging control means for stopping the charging at a charge level less than a full-charge level; second charging control means for stopping the charging at a charge level equal to or more than the full-charge level and deciding means for deciding that the charging by the first charging control means is continuously repeated by a specific number of times. A control means switching means is provided for switching, if it is decided by the deciding means that the charging by the first charging control means is continuously repeated by the specific number of times, the first charging control means to the second charging control means so as to perform the next charging by the second charging control means.
According to another feature of the present invention, there is provided a system for controlling the charging of a secondary battery, including: first charging control means for stopping the charging at a charge level less than a full-charge level; second charging control means for stopping the charging at a charge level equal to or more than the full-charge level and a random number generating means for generating a random number each time one charging by the first charging control means is performed. A control means switching means is provided for switching, if the random number generated by the random number generating means corresponds to a specific value equal to or less than the maximum value among the generated random numbers, the first charging control means to the second charging control means so as to perform the next charging by the second charging control means.
According to another feature of the present invention, the above control means switching means is configured such that the specific number of times of the repeated charging by the first charging control becomes less than 20.
According to a further feature of the present invention, the battery capacity, less than the full-charge level, at which the charging by the first charging control means is to be stopped, is composed of two-step battery capacities. A selecting means is provided for selecting either of the two-step battery capacities.
According to a still further feature of the present invention, the system further includes display means, wherein if a lower one of the two-step battery capacities is selected by the selecting means and the second charging control means is selected by the control means switching means, the display means displays a signal indicating that the selection is canceled.
According to the above features, after the first charging control specified to stop the charging at a charge level less than a full-charge level is continuously repeated by a specific number of times, the second charging control specified to stop the charging at a charge level more than the full-charge level is performed. In particular, according to the fourth feature, the control is performed such that the charging by the first charging control is not continuously repeated by 20 times or more.
According to one of the features of the present invention, the charging control can be selected such that the charging is stopped at a battery capacity, which is less than and apart from a full-charge level, and which is set before the charging reaches a battery capacity less than and close to the full-charge level. According to a feature of the invention, when the charging must be performed by the second charging control although the lower battery capacity has been selected by the fifth feature, such a state can be displayed, to thereby eliminate an inconvenience that a user is distrustful of the state.
According to one of the features of the present invention, the above refresh discharge reference is determined to be satisfied when the remaining capacity of the secondary battery is less than a specific value. According to a further feature of the present invention, the refresh discharge reference is determined to be satisfied each time the charging by the second charging control is repeated by a specific number of times.
If the charging of a secondary battery at a charge level at less than a full-charge level is repeated, it becomes impossible to ensure an initial charging/discharging characteristic of the secondary battery by a memory effect. To solve such a problem, according to the features of the present invention, after the first charging control specified to stop the charging at a charge level less than a full-charge level is continuously repeated by a specific number of times, the second charging control specified to stop the charging at a charge level more than the full-charge level is performed. Further, if the state of the secondary battery satisfies a refresh discharge reference, the refresh discharging is performed before the charging of the secondary battery to a charge level more than the full-charge level is performed. As a result, it is possible to efficiently eliminate the memory effect. In particular, according to the features of the present invention, it is possible to decide the discharging timing of the secondary battery on the basis of a simple refresh discharge reference. In addition, the charging by the second charging control is performed by a preceding charging performed at a second charging current larger than the first charging current and a subsequent charging performed at a third charging current smaller than the first charging current.
According to a feature of the present invention, a system is provided for controlling the charging of a secondary battery, including: cut-off voltage storing means for setting a cut-off voltage corresponding to a specific capacity less than a full-charge level of the second battery; first charging control means for charging the secondary battery at a first charging current and stopping the charging when a charging voltage of the secondary battery reaches the cut-off voltage; second charging control means for charging the secondary battery at a second charging current larger than the first charging current until the charging voltage of the secondary battery reaches the cut-off voltage, and then charging the secondary battery at a third charging current smaller than the first charging current until the charge level of the secondary battery reaches the full-charge level; and switching means for selecting either the first charging control means or the second charging control means.
According to a feature of the present invention, the above system further includes full-charge voltage storing means for storing a full-charge voltage of the secondary battery when the charging by the second charging control means is terminated; and correcting means for correcting a deteriorated proportion of the cutoff voltage on the basis of the value of the full-charge voltage stored in the full-charge voltage storing means.
According to a fourth feature of the present invention, the above system further includes full-charge voltage storing means for storing a full-charge voltage of the secondary battery when the charging by the second charging control means is terminated; setting means for setting a reference full-charge voltage at a specific temperature of the secondary battery and temperature storing means for storing a temperature of the secondary battery at the time when the full-charge voltage is stored in the full-charge voltage storing means. Correcting means are provided for correcting a deteriorated proportion of the cut-off voltage on the basis of the value of the full-charge voltage stored in the full-charge voltage storing means, the value of the temperature of the secondary battery stored in the temperature storing means and the reference full-charge voltage.
If the charging of a battery to a charge level less than the full-charge level is repeated, it becomes impossible to ensure an initial charging/discharging characteristic of the battery due to the memory effect. To solve such a problem, according to the features of the invention, after the first charging control specified to stop the charging at a charge level less than the full-charge level is continuously repeated by a specific number of times, the second charging control specified to stop the charging at a charge level more than the full-charge level is performed.
In particular, according to a feature of the invention, in the second charging control, since the preceding charging is performed at a large current, the charging time is shortened, and the subsequent charging performed at a charge level near the full-charge level is slowly performed, with a result that the total charging time can be made nearly equal to that required for the first charging control.
According to a feature of the present invention, since the charging current switching timing in the second charging control can be based on a cut-off voltage value in the first charging control, the control can be simply performed without the provision of any special decision means.
According to other features of the present invention, to cope with an inconvenience wherein the full-charge voltage by the second charging control is changed due to a deterioration of the secondary battery, the degree of deterioration of the secondary battery is decided on the basis of the stored value of the full-charge voltage, and the decision result can be reflected on the reference full-charge voltage. In particular, according to a feature of the present invention, the cut-off voltage can be corrected on the basis of not only the stored value of the full-charge voltage but also the stored value of the temperature of the secondary battery at the time of charging the battery to the full-charge level.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.